Polyimide film is useful in a variety of applications, because it has excellent chemical resistance and mechanical properties throughout a wide range of temperatures. These and other properties make polyimide film particularly suitable as insulation for the protection of power and signal wire. However, during installation, maintenance or service, insulation on one or more wires installed as a bundle may become damaged by chafing, chemical contact, or other means in a way that exposes the conductor. Electrical arcing can occur when the conductor of a damaged wire in the bundle comes into contact with another conductor or a grounded surface. The resulting short-circuit and subsequent reenergizing of the wire may cause further electrical arcing which produces localized areas of very high temperatures. Rapid thermal degradation of the insulation material may then occur. This degradation is called "arc tracking" (also commonly referred to as arc propagation), wherein electrical arcing is perpetuated throughout the insulation material. The arc tracking may damage the electrical system. Arc track resistant polyimide insulation does not propagate damage caused by this type of event.
U.S. Pat. No. 3,424,718 to Angelo concerns a copolyimide of an aromatic tetracarboxylic acid and at least three different diamines, two of said diamines being aromatic diamines and the other of said diamines being an aryl aliphatic diamine having an alkylene group. However, this reference does not disclose copolyimides containing aliphatic diamines as having arc track resistance.
U.S. Pat. No. 3,551,200 and related U.S. Pat. No. 3,677,921 to Stivers relate to thermoplastic polyimides of straight chain alkylene diamines and pyromellitic anhydride for use in wire insulation and film as a base for printed circuits However, these references do not describe copolyimides as having the unique combination of arc track resistant properties and thermal aging stability as in the present invention.
U.S. Pat. No. 3,803,103 to Magay is directed to copolyimides derivable from straight chain and branched chain diamines and tetracarboxylic acid moieties and used as electrical insulation. However, this reference does not reveal a composition as having arc track resistance.
Thus, there is a need for polyimide based insulating materials that are not susceptible to arc tracking. Such materials would preferably additionally exhibit the desirable mechanical properties and chemical resistance characteristic of polyimides. Moreover, such compositions must be readily formable as protective laminates, films, or coatings.
It is an object of the present invention to provide a polyimide based film that exhibits arc track resistance. It is a further object of this invention to provide a polyimide based film that has good heat aging stability. A still further object of this invention is to provide a polyimide based composition that substantially retains the beneficial properties of the polyimide, including tensile strength, elongation, and other properties. A feature of the present invention is the unique combination of aromatic and aliphatic diamines together with an aromatic dianhydride to furnish a copolyimide exhibiting these properties. An advantage of the present polyimide based film is that it can be formed as a laminate around power cable and wire to provide a protective arc track resistant sheath. These and other objects, features, and advantages will become readily apparent from the following description of the invention.